The present invention relates to an electric capacitance strain gauge and particularly to a technique for unifying the gauge characteristics of the electric capacitance strain gauge as much as possible.
The conventional electric capacitance strain gauge is shown in FIG. 1. The electric capacitance strain gauge shown in FIG. 1 is a strain gauge described at pages 5 to 8 of the 19th Stress and Strain Measurement Symposium Lecture Paper Collection of the Japan Non-destructive Inspection Association.
The constitution of the electric capacitance strain gauge shown in FIG. 1 is as follows.
The installing frames 2 and 3 made of heat resisting conductive metal cross three-dimensionally without touching each other at the central part. Two pairs of electrode plates 5a, 5b, 6a and 6b are fixed on the opposing inner surfaces of the installing frames 2 and 3 with an electrically insulated ceramic adhesive material 4. A fulcrum spring 7 is spot welded at the right ends of the installing frames 2 and 3. Incidentally, the left end of the electric capacitance strain gauge is attached to a measured material 1 by spot welding.
In the case where the measured material 1 is extended, the space between the electrode plates 5a and 5b constituting a pair becomes large and conversely, space between the electrode plates 6a and 6b constituting another pair becomes small, which makes the polarity of the electric capacitance change. Then, the electric capacitance change is added and measured with each of the electrode pairs adjacent to an electrically balanced circuit. In addition, the temperature is compensated for as an electric capacitance change caused by other environmental factors (for example, temperature change).
In addition, the gauge factor Ks, which is the fundamental characteristic of the electric capacitance strain gauge, is expressed as follows using dimensions lA, lD, GL, D.sub.A, D.sub.D and L. ##EQU1## And it can be decided from the dimensional shape of the gauge set at the beginning. Therefore, if the electrode plates 5a, 5b, 6a and 6b are not installed accurately on the installing frames 2 and 3, the fundamental characteristic of the gauge is not known until each dimension of the gauge is measured again after fixing the electrode plates 5a, 5b, 6a and 6b on the installing frames 2 and 3 since the dimensions differ in each gauge.
Here the rear faces of the electrode plates 5a, 5b, 6a and 6b are fixed on the installing frames 2 and 3 with an electrically insulated ceramic adhesive material 4. But trouble occurs in that it is impossible to make the thickness of the adhesive a constant amount as the ceramic adhesive material cannot be applied uniformly on the electrode plates 5a, 5b, 6a and 6b, that they cannot be adhered at room temperature and heat setting processing is necessary, but temperature management is difficult and the yield is not satisfactory, and that the insulation resistance of the adhesive material is lowered at high temperature in general and becomes unusable (if the thickness of the adhesive is increased, the insulation characteristic is improved, but air bubbles are generated and adhesive strength is lowered), which limits the working temperature to about 600.degree. C.
As it is almost impossible to accurately install the electrode plates 5a, 5b, 6a and 6b at the specified positions of the installing frames 2 and 3, it is impossible to mass-produce them with a constant gauge factor, which is the fundamental characteristic of the gauge. It is also difficult to measure the dimensions of each part with accuracy after installing the electric capacitance strain gauge on the measured material 1, and it is impossible to measure the strain accurately.
There is a problem in the above conventional technique in that dimnesional accuracy for installing the electrode plates is not considered and those with different characteristics for each gauge are produced, which reduces the strain measurement accuracy, and that electrode plate installation strength is not considered either, and which lacks reliable strength.